Method of and system for deciding failures of automatic transmission

ABSTRACT

A gear shift failure of an automatic transmission to a wrong gear other than an indented gear is made by estimating slippage theoretically caused in a torque converter on the basis of an engine speed and a turbine speed theoretically determined for an intended gear according to the vehicle speed and comparing the estimating slippage with predetermined reference slippage. When the estimated slippage exceeds the predetermined threshold slippage, it is decided that there has occurred a wrong shift to a gear other than the intended gear.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of and a system for detecting afailure of an automatic transmission for an automobile, and, moreparticularly, to a method of and a system for making a decision on afailure of an automatic transmission in gear shift to an intended gear.

2. Description of Related Art

A multiple speed transmission gear mechanism of an automatictransmission equipped with a torque converter is typically connected toan engine through the torque converter. Gear shifts of the automatictransmission are completed by selectively actuating a plurality ofelectromagnetic solenoids to hydraulically lock and unlock frictioncoupling elements, such as brakes and clutches, so as to create desiredpower transmission paths of the multiple transmission gear mechanism.Specifically, control signals are generated and selectively actuate theelectromagnetic solenoids according to predetermined gear shift patternsselected on the basis of driving conditions.

Automatic transmissions of this kind possibly encounter failures that anexpected or target gear is not created for some reasons or others. Adecision on an occurrence of such a shift failure, which is always oneof matters of importance for safety drive, can be made by a comparisonbetween a theoretical turbine speed, theoretically determined based on avehicle speed and a target gear, and an actual turbine speed. If theautomatic transmissions are equipped with turbine speed sensors, thefailure decision is easy and accurate. However, in some cases where theautomatic transmission is not equipped with a turbine speed sensor, amatter of great importance is how to decide an occurrence of a shiftfailure and how accurately the failure decision is made.

One of approaches to making a failure decision is to estimate a turbinespeed on the basis of an engine speed and a speed ratio e of the torqueconverter. The torque converter speed ratio e, i.e. a speed ratiobetween a turbine speed and a pump speed, is obtained from a speed ratiomap established in relation to input torque coefficient T as a parameterwhich is an engine output torque T divided by a square of the enginespeed NE. In this instance, the engine output torque T is obtained froma torque map established in relation to engine throttle opening TVO andengine speed NE. Such a technique is known from, for instance, JapaneseUnexamined Patent Publication No. 6(1994)-331020.

This technique taught by the Japanese Unexamined Patent Publication No.6(1994)-331020 does, however, not always provide an accurate andreliable decision on an occurrence of a failure in gear shift. This isbecause the estimate for a turbine speed necessities a number ofmathematical calculations and experiments errors in various parameters,such as the temperature and pressure of an operating oil in the multipletransmission gear mechanism.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of makingan accurate and reliable decision on a failure in shifting an automatictransmission equipped with no a turbine speed sensor to a target gear.

It is another object of the present invention to provide a failuredecision system for making a decision on a failure in shifting anautomatic transmission equipped with no a turbine speed sensor to atarget gear.

These objects of the present invention are achieved by providing afailure decision system for making a decision of operational failure ofan automatic transmission which is comprised of a torque converter and amultiple speed transmission gear mechanism, the automatic transmissionautomatically shifting to target gears selected according topredetermined shift patterns on the basis of driving conditions. In thefailure decision, slippage theoretically caused in said torque converteris estimated on the basis of an engine speed and a turbine speed of thetorque converter theoretically determined for a target gear selectedaccording to the vehicle speed, and is compared with predeterminedreference slippage. When the estimated slippage exceeds thepredetermined threshold slippage, it is decided that there has occurreda wrong shift to a gear other than the target gear.

Slippage actually caused in the torque converter does not exceed thetheoretical maximum slippage which alters according to vehicle speedsand also according to gears even when the vehicle speed does not change.If the automatic transmission shifts to a wrong gear other than thetarget gear, for instance to a gear lower than the target gear, theestimated slippage exceeds the maximum slippage. On the basis of thisfact, a gear shift failure is precisely and easily found with theutilization of the maximum slippage as a reference or threshold value.

On the other hand, if the automatic transmission shifts to a wrong gearhigher than the target gear, the estimated slippage is attained as aminus value which is the difference of an engine speed from an estimatedturbine speed which is higher than the engine speed. On the basis ofthis fact, with the utilization of the minus slippage as a reference orthreshold value, a gear shift failure is precisely and easily found.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbe clearly understood from the following description with respect to apreferred embodiment thereof when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram of a failure decision system in accordancewith a particular embodiment of the present invention;

FIGS. 2 through 7 are a flowchart illustrating a sequence routine ofexecuting the method of making a failure decision in accordance with aspecific embodiment of the invention;

FIG. 8 is a flowchart illustrating a sequence subroutine of the decisionas to whether a gear shift is taking place;

FIG. 9 is a graph of maximum slippage allowable for the torque converterto cause;

FIG. 10 is a graph for explaining the principle of making the failuredecision;

FIG. 11 is part of a flwochart illustrating a sequence routine ofexecuting the method of making a failure decision in accordance withanother specific embodiment of the invention; and

FIG. 12 is a flwochart illustrating a sequence routine of establishingthreshold vehicle speeds in the failure decision sequence routine shownin FIG. 11.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Because multiple transmission gear mechanisms and torque converters arewell known in the art, the present description will be directed inparticular to elements forming part of, or cooperating directly with, anapparatus or a system in accordance with the present invention. It is tobe understood that elements not specifically shown or described can takevarious forms well known to a person skilled in the automobile art.

Referring to the drawings in detail, and more particularly to FIG. 1, anautomatic transmission AT is comprised of a multiple transmission gearmechanism 3 of a type having a planetary gearset and a torque converter2 through which the multiple transmission gear mechanism 3 is connectedto an engine 1 and which differentially transmits the engine outputtorque to the multiple transmission gear mechanism 3. This torqueconverter 2 is equipped with a hydraulically operated lockup clutch 4for mechanically coupling input and output shafts, i.e. an engine outputshaft to which the pump is fastened and a transmission input shaft towhich the turbine is fastened, together. The lockup clutch 4 is lockedand unlocked by turning on and off a lockup control electromagneticvalve R1. Multiple transmission gear mechanism 3 includes a plurality ofhydraulically operated friction coupling elements, such as brakes andclutches, which are selectively locked and unlocked to create variouspower transmission paths and thereby to shift the automatic transmissionAT to desired forward gears. Locking and unlocking of the frictioncoupling elements are caused by various predetermined combinations ofactuation of shift control electromagnetic valves R2-R4. The automatictransmission AT is of, for instance, having four forward gears and areverse gear. A control unit U comprised mainly of a microcomputer Ureceives signals from various sensors and controls actuation of theseelectromagnetic valves R1-R4. These sensors include at least a speedsensor S1 for detecting a rotational speed NE of the engine 1 andproviding an engine speed signal, a load sensor S2 for detecting aposition or opening TVO of an engine throttle as an engine load andproviding a position signal, a speed sensor S3 for detecting a vehiclespeed NV and providing a vehicle speed signal, a temperature sensor S4for detecting the temperature THO of an operating oil in the Automatictransmission AT, and an idle sensor S5 for detecting an idle state ofthe engine 1. The vehicle speed sensor S3 may practically detect therotational speed of driven wheels, or otherwise the rotational speed ofan output shaft 5 of the automatic transmission AT. The idle sensor S5may be comprised of a switch which turns on following releasing anacceleration pedal.

Control unit U is comprised of functional sections, such as a shiftdecision section 10, a lockup decision section 11, a shift failuredecision section 12 and a lockup failure decision section 13, into whichthe microcomputer is divided not practically but conceptually. Thecontrol unit U further includes memories such as a read only memory(ROM) and a random access memory (RAM) for storing data on lockupcontrol patterns and maximum slippage allowable for the torque converter2 to cause according to vehicle speeds and gears and other necessarydata.

Shift decision section 10 decides a target gear to which the automatictransmission AT must be shifted on the basis of predetermined gear shiftpatterns and provides the shift control electromagnetic valves R2-R4with shift command signals. The gear shift patterns define gears to beselected according to combinations of, for instance, throttle openingand vehicle speeds as parameters. The lockup decision section 11 decideswhether or not the lockup clutch 4 must be locked on the basis of apredetermined lockup pattern and provides the lockup controlelectromagnetic valve R1 with a lockup command signal. For example, thelockup pattern defines a lockup range of vehicle speeds for locking thelockup clutch 4 on condition that the automatic transmission AT is in,for instance, a fourth gear.

Shift failure decision section 12 makes a decision as to whether theautomatic transmission AT has successfully been shifted to a targetgear. In the case where there is an occurrence of a wrong gear shift toa gear other than the target gear, the shift failure decision section 12provides a failure signal with which a warning device 21, such as abuzzer and a lump, raises a warning. The lockup failure decision section13 makes a decision of an occurrence of a failure that the lockup clutch4 is left unlocked even at the presence of a lockup command signal ispresent and, if in fact there is an occurrence of a failure of thelockup clutch 4, provides a failure signal with which a warning device22, such as a buzzer and a lump, raises a warning.

The principle of the failure decision will be attained by reference madeto FIGS. 9 and 10. FIG. 9 shows a map of maximum slippage which thetorque converter 2 is allowed to produce at various vehicle speeds. Themaximum allowable slippage map is provided for each available gear andestablished such that the maximum allowable slippage declines with anincrease in vehicle speed. FIG. 10 shows changes in engine speed NE andturbine speed NT in relation to vehicle speeds NV. As shown, slippagerepresented by a difference of the engine speed NE from the turbinespeed NT becomes smaller with an increase in vehicle speed NV andattains a small and constant value in a range of vehicle speeds NVhigher than a certain vehicle speed. The turbine speed NT for a specificgear is theoretically obtained as an estimated turbine speed NT on thebasis of a vehicle speed NV and a gear ratio of the whole drive systemwhen the automatic transmission AT is placed in the specific gear.

When estimated slippage at a vehicle speed NV obtained as the differencebetween an engine speed NE and an estimated turbine speed NT is greaterthan the maximum slippage at the vehicle speed NV, it is assumed thatthe automatic transmission AT has been shifted to a wrong gear differentfrom the target gear due to a some failure or other. In this failure,the wrong gear is slower than the target gear. Specifically, consideringthat the automatic transmission AT has practically been shifted not to afourth gear as the target gear but to a third gear when the vehicle isrunning at a speed greater than a specific vehicle speed NVs, namely avehicle speed NVs₄ assigned to the fourth gear, as shown in FIG. 10which will be described in detail later, the engine speed NE accords tothe practically provided third gear and is greater than that attained inthe fourth gear, and the estimated turbine speed NT in the target fourthgear is significantly smaller than the turbine speed NT in the thirdgear. In such a way, in cases where a gear practically created is lowerthan a target gear, because of a larger engine speed NE and a smallerestimated turbine speed NT, the estimated slippage exceeds greatly overthe maximum allowable slippage shown in FIG. 9. On the other hand, incases where a gear practically created is higher than a target gear, theestimated slippage takes a minus value. That is, an occurrence of theestimation of a minus value of slippage signifies a wrong shift to agear higher than the target gear. For instance, if a gear shift occursto a fourth gear different from the target gear, for instance a thirdgear, whenever the vehicle is running at a speed greater than thespecific vehicle speed NVs, namely a vehicle speed NVs₃ assigned to thethird gear, the engine speed NE is lower according to the fourth gearand the estimated turbine speed NT is greater according to the targetthird gear for which the estimated slippage takes a minus value.

The specific vehicle speed NVs is a speed provided when the turbinespeed at a certain target gear, for instance the third gear, is equal tothe engine speed when the automatic transmission AT is placed at a gearone step higher than the target gear. Because the engine speed NE at thefourth gear is higher than the turbine speed at the third gear, if thevehicle speed is lower than the specific speed NVs, an accurate andreliable failure decision is hard to take place normally. In such cases,the execution of failure decision and withdrawal of the decision offailure are interrupted whenever the vehicle speed is in a range lessthan the specific vehicle speed NVs.

The operation of the failure decision system depicted in FIG. 1 is bestunderstood by reviewing FIGS. 2-5, which are a flowchart illustrating asequence routine of the shift failure decision for the microcomputer, inparticular the shift failure decision section 12, of the control unit U.Programming a computer is a skill well understood in the art. Thefollowing description is written to enable a programmer having anordinary skill in the art to prepare an appropriate program for themicrocomputer. The particular details of such a program would of coursedepend upon the architecture of the particular computer selected.

The flowchart logic commences and control passes directly to a functionblock at step Q1 where various signals from the sensors S1-S5, whichrepresent a vehicle speed NV, an engine speed NE, throttle opening TVOand an oil temperature THO, respectively, are input. After adetermination of a target gear TG according to an appropriate shiftpattern selected based on the vehicle speed NV and throttle opening TVOat step Q2, various threshold values, namely first to fourth thresholdtimes T1-T4 and first and second threshold slippage DS1 and DS2, aresubsequently determined according to the target gear TG at steps Q3through Q9. These threshold times T1-T4 are used to make a failuredecision. Specifically, the threshold times T1 and T3 are used inconnection with a decision on a wrong shift to a gear lower than thetarget gear and a withdrawal of the decision of failure, respectively.Similarly, the threshold times T2 and T4 are used for a failure decisionon a wrong shift to a gear higher than the target gear and a withdrawalof the decision of failure, respectively. In consideration of a time forwhich each gear is continuously used, each threshold time T1-T4 isgenerally established to be shorter as the target gear becomes lower.Specifically, in this embodiment, while the threshold times T1 and T2are invariable, the threshold times T3 and T4 are variable such thatthey becomes shorter with an increase in vehicle speed NV and with anincrease in throttle opening TVO. The threshold values DS1 and DS2,which, as was previously described, indicate the maximum allowableslippage as shown in FIG. 9, are used for making a decision of a wrongshift to a gear lower than a target gear and a decision of a wrong shiftto a gear higher than a target gear, respectively. The thresholdslippage DS2, which takes a minus value, is established to be slightlysmaller as shown by a dotted broken line in FIG. 9 in order to avoidwrong decisions.

When the first gear (1st) is selected as the target gear at step Q3, thethreshold values T1-T4 and DS1 and DS2 are set at step Q4. Specifically,timers TS11 and TS12 are set to the threshold times T1 and T2 for makinga failure decision concerning the target first gear (1ST), and timersTR11 and TR12 are set to threshold times T3 and T4 for withdrawing thedecision of failure concerning the target first gear (1ST). Together,the threshold slippage DS1 and DS2 are set to maximum slippage asfunctions of vehicle speed f₁₁ (NV) and f₁₂ (NV) assigned to the targetfirst gear. In the same manner, the threshold values T1-T4 and DS1 andDS2 are set at step Q6 when the second gear (2ND) is selected as thetarget gear at step Q5, at step Q8 when the third gear (3RD) is selectedas the target gear at step Q7, and at step Q9 when the fourth gear (4TH)is selected as the target gear.

Thereafter, decisions are consecutively made regarding requiredconditions for the failure detection at steps Q10 through Q12.Specifically, a subroutine shown in FIG. 8 is called at step Q10 to makea decision as to whether a gear shift to the target gear TG is takingplace. If the answer is "NO," another decision is made at step Q11 as towhether the throttle opening TVO is equal to or less than a limit valueTVOx which is exceeded during deceleration. In place of throttleopening, it may be decided as to whether the idle switch S5 has beenturned on. If the answer is "NO," a further decision is made at step Q12as to whether the oil temperature THO is lower than a limit value THOx.The oil temperature is one of factors governing a period of time forachieving a gear shift. If the answer is "NO," the shift failuredecision sequence routine proceeds to further steps. However, if any oneof the required conditions is not satisfied, the shift failure decisionsequence routine returns without executing the failure decision.

At steps Q13 through Q25 in FIG. 3, decisions are made regarding furtherrequired conditions for the failure detection. Specifically, in the casewhere the first gear (1ST) is selected as the target gear (TG), theshift failure decision is executed only when the vehicle speed NV andthe throttle opening TVO exceed a first specified vehicle speed NVX1 anda first specified throttle opening TVOX1, respectively, which are lowerlimits for allowing execution of the failure decision at the first gear(1ST). The first vehicle speed NVX1 is preferred to be slightly higherthan the specific vehicle speed NVs at the first gear shown in FIG. 10.In the case where the second gear (2ND) is selected as the target gear(TG), the shift failure decision is executed only when the vehicle speedNV and the throttle opening TVO exceed a second lower limit vehiclespeed NVX2 and a second lower limit throttle opening TVOX2,respectively. The second lower limit vehicle speed NVX2 is alsopreferred to be slightly higher than the specific vehicle speed NVs atthe second gear. If these conditions on vehicle speed and throttleopening are not satisfied, the shift failure decision sequence routinereturns without executing the shift failure decision. In the case wherethe third gear (3RD) is selected as the target gear (TG), the shiftfailure decision is executed only when the vehicle speed NV exceeds athird lower limit vehicle speed NVX3 and the throttle opening TVO isbetween a third lower limit throttle opening TVOX31 and a first upperlimit throttle opening TVOX32.

Further, in the case where the fourth gear (4TH) is selected as thetarget gear (TG), the shift failure decision is executed only when thevehicle speed NV exceeds a fourth lower limit vehicle speed NVX4 and thethrottle opening TVO is between a fourth lower limit throttle openingTVOX41 and a second upper limit throttle opening TVOX42. The upper limitthrottle openings are used to conduct the failure decision only in arange of drive conditions which are frequently satisfied at the thirdgear and at the fourth gear, respectively.

In this instance, the lower the target gear is, the greater the limitthrottle opening is, and, on the other hand, the higher the target gearis, the greater the lower limit vehicle speed is.

When all of the required conditions for the failure detection aresatisfied, a decision is made as to whether a wrong shift to a gearlower than the target gear has occurred. Specifically, at step Q26 inFIG. 4, current estimated slippage SLP_(TG)(n) allowable for the targetgear TG is calculated as a difference between engine speed NE andestimated turbine speed NT from the following equation:

    SLP.sub.TG =NE-NV×K.sub.TG

where K_(TG) is the coefficient assigned to the specific target gear TGand corresponds to a gear ratio of the entire drive system when thetarget gear TG is selected.

Subsequently, a decision is made at step Q27 as to whether the currentestimated slippage SLP_(TG)(n) is greater than the second thresholdvalue DS2. If greater, another decision is made at step Q28 as towhether the current estimated slippage SLP_(TG)(n) is greater than thefirst threshold value DS1. If the answer to the decision made at stepQ28 is "YES," this indicates that the gear shift has been wrongly madeto a lower gear, then, a decision is subsequently made at step Q29 as towhether a wrong shift flag DG has been set to a state of "0". In thisinstance, the wrong shift flag DG set to the state of "0" indicates thatthere has occurred no wrong shift to a lower gear. If the answer to thedecision made at step Q29 is "YES," then, a decision is made at step Q30as to whether the last estimated slippage SLP_(TG)(n-1) is greater thanthe first threshold value DS1. If the answer to the decision is "NO,"this indicates that the first threshold value DS1. If the answer to thedecision is "NO," this indicates that the first threshold value DS1 iscurrently exceeded for the first time, then, the timer TS1 (TS11, TS21,TS31 or TS41) resets its initial count to the shift failure decisionthreshold time T1 at step Q31. On the other hand, if the answer to thedecision is "YES," this indicates that the first threshold slippage hasalready been exceeded, then, the timer TS1 changes its count by adecrement of one at step Q32. Subsequently, a decision is made at stepQ33 as to whether the timer TS1 has counted down the shift failuredecision threshold time T1 to zero. At an early time, since the answerto the decision is "NO," the shift failure decision sequence routineproceeds to further steps. When the answer to the decision at step Q33is "YES," this indicates that a wrong shift to a lower gear hascontinued for the shift failure decision threshold time T1, then, thewrong shift flag DG is set to a state of "1" at step Q34. If the answerto the decision made at step Q27 or at step Q29 is "NO," then, the shiftfailure decision sequence routine proceeds to further steps skippingsteps Q28-Q34 or steps Q30-Q34, respectively.

If the first threshold slippage DS1 is exceeded, that is, the answer tothe decision made at step Q28 is "YES," a decision is made at step Q35as to whether the wrong shift flag DG has been set to the state of "1".If the answer to the decision is "NO," this indicates that withdrawal ofthe decision of failure is not necessary, then, the shift failuredecision sequence routine proceeds to further steps skipping stepsQ36-Q40. On the other hand, if the answer to the decision is "YES," theshift failure decision time is appropriately set through subsequentsteps Q36-Q40. Specifically, a decision is made at step Q36 as towhether a timer TR1 (TR11, TR21, TR31 or TR41) has counted down theshift failure withdrawal threshold time T3 to zero. After having resetthe timer TR1 to the shift failure withdrawal threshold time T3 at stepQ37 if the answer to the decision at step Q36 is "YES," or after havingchanged the count of the timer TR1 by a decrement of one at step Q38 ifthe answer to the decision at step Q36 is "NO," a decision is made atstep Q39 as to whether the timer TR1 has counted down to zero. At anearly time, since the answer to the decision is "NO," the shift failuredecision sequence routine proceeds to further steps. When the answer tothe decision at step Q39 is "YES," the wrong shift flag DG is reset tothe state of "0" at step Q40. Withdrawal of the decision of failure, inother words resetting the wrong shift flag DG to the state of "0," isexecuted through steps Q36-Q40 when slippage SLP_(TG) is continuouslyestimated to be less than the first threshold slippage DS1 for the timeTR1.

Thereafter, at steps Q41 through 57 in FIG. 5, a decision concerning awrong shift to a higher gear is made, in other words, setting orresetting a wrong shaft flag UG relating to a wrong shift to a gearhigher than a specific target gear is executed. Specifically, a decisionis made at step Q41 as to whether the estimated slippage SLP_(TG)allowable for the target gear is greater than the first thresholdslippage DS1. If smaller, another decision is made at step Q42 as towhether the estimated slippage SLP_(TG) is greater than the secondthreshold slippage DS2. If the answer to the decision made at step Q42is "NO," a decision is subsequently made at step Q43 as to whether thewrong shift flag UG has been set to a state of "0". In this instance,the wrong shift flag UG set to the state of "0" indicates that there hasbeen no occurrence of a wrong shift to a higher gear. If the answer tothe decision made at step Q43 is "YES," then, a decision is made at stepQ44 as to whether the last estimated slippage SLP_(TG)(n-1) is greaterthan the second threshold value DS2. If the answer to the decision is"YES," the timer TS2 (TS12, TS22, TS32 or TS42) resets its initial countto the shift failure decision threshold time T2 at step Q45. On theother hand, if the answer to the decision is "NO," the timer TS2 changesits count by a decrement of one at step Q46. Subsequently, a decision ismade at step Q47 as to whether the timer TS2 has counted down the shiftfailure decision threshold time T2 to zero. If the answer to thedecision at step Q47 is "YES," this indicates that a wrong shift to ahigher gear has continued for the shift failure decision threshold timeT2, then, the wrong shift flag UG is set to the state of "1" at stepQ48. After having set the wrong shift flag UG to the state of "1" atstep Q48 or if the answer to the decision made at step Q47 is "NO," theshift failure decision sequence routine proceeds to a decision at stepQ55. Also, the "YES" answer to the decision at step Q41 or the "NO"answer to the decision at step Q43 calls directly a decision at step Q55skipping steps Q42-Q54 or steps Q44-Q48, respectively. Summarizing theabove operation caused through steps Q42-Q48, when the slippage SLP_(TG)is continuously estimated to be less than the second threshold slippageDS2 which assumes a minus value for the shift failure decision thresholdtime T2, the wrong shift flag UG is set to the state of "1" whichindicates an occurrence of a wrong shift to a gear higher than thetarget gear.

If the second threshold slippage DS2 is exceeded, that is, the answer tothe decision made at step Q42 is "YES," a decision is made at step Q49as to whether the wrong shift flag UG has been set to the state of "1".If the answer to the decision is "NO," the shift failure decisionsequence routine proceeds directly to step Q55 skipping steps Q50through Q54. On the other hand, if the answer to the decision is "YES,"a decision is made at step Q50 as to whether a timer TR2 (TR12, TR22,TR32 or TR42) has counted down to zero. After having reset the timer TR2to the shift failure withdrawal threshold time T4 at step Q51 if theanswer to the decision at step Q50 is "YES," or after having changed thecount of the timer TR2 by a decrement of one at step Q52 if the answerto the decision at step Q50 is "NO," a decision is made at step Q53 asto whether the timer TR2 has counted down the shift failure withdrawalthreshold time T4 to zero. If the answer to the decision is "NO," theshift failure decision sequence routine proceeds directly to step Q55.When the answer to the decision at step Q53 is "YES," the wrong shiftflag UG is set to the state of "0" at step Q54. Summarizing the aboveoperation caused through steps Q49-Q54, when the slippage SLP_(TG) iscontinuously estimated to be greater than the second threshold value DS2which assumes a minus value for the shift failure withdrawal thresholdtime T4, the wrong shift flag UG is set to the state of "0" whichindicates that there is no occurrence of a wrong shift to a gear higherthan the target gear.

Finally, decisions are made as to the states of the wrong shift flags DGand UG at steps Q55 and Q56, respectively. At least one of the wrongshift flags DG and UG assumes the state of "1," at step Q58, a shiftfailure flag G is set to a state of "1" which actuates the warningdevice 21 to indicate an occurrence of a shift failure of the automatictransmission. On the other hand, if both wrong shift flags DG and UGassume the state of "1" at step Q58, the shift failure flag G is set toa state of "1" which indicates that there is no occurrence of a shiftfailure of the automatic transmission.

FIGS. 6 and 7 are a flowchart illustrating a sequence routine of thelockup failure decision for the microcomputer, in particular the lockupfailure decision section 13, of the control unit U.

The first step in FIG. 6 is to calculate estimated slippage SLP1-SLP4allowable for the first to fourth gears G1-G4, respectively, at step P1,following step Q22 or Q25 in FIG. 3.

The estimated slippage SLPn allowable for the specific gear Gn iscalculated as a difference between engine speed and estimated turbinespeed from the following equation:

    SLPn=NE-NV×Kn

where K is the coefficient assigned to the specific gear Gn andcorresponds to a gear ratio of the entire drive system when the gear Gnis selected.

Subsequently, a decision is made at step P2 as to whether the drivingcondition is within a predetermined lockup (L/U) region. If it is out ofthe lockup (L/U) region, the lockup failure decision sequence routinereturns without executing further steps for the lockup failure decision.If it is within the lockup (L/U) region, a decision is made at step P3as to whether a lockup command signal provided for the lockup controlelectromagnetic valve R1 has a duty rate of 100%. In this instance, thelockup command signal of a 100% duty rate causes the lockup controlelectromagnetic valve R1 to operate so as to lock perfectly the lockupclutch 4. If it has not a 100% duty rate, the lockup failure decisionsequence routine returns without executing further steps for the lockupfailure decision.

If in fact the lockup command signal has a 100% duty rate, decisions aremade as to whether the current estimated slippage SLP1.sub.(n)-SLP4.sub.(n) are greater than a threshold value DS at steps P4 throughP7, respectively. If all of the estimated slippage SLP1-SLP4 are greaterthan the threshold value DS, this indicates that the lockup clutch 4produces some slippage, then, a decision is made at step P8 as towhether a lockup failure flag LU has been set to a state of "0" whichindicates that the lockup clutch 4 is successfully locked up. If theanswer to the decision is "YES," decisions are made at steps P9 throughP12 as to whether the last estimated slippage SLP1.sub.(n-1)-SLP4.sub.(n-1) are greater than the threshold value DS at steps P9through P12, respectively. If any one of the last estimated slippageSLP1.sub.(n-1) -SLP4.sub.(n-1) is less than the threshold value DS, thisindicates that a lockup failure has currently occurred for the firsttime, then, a timer TL5 resets its initial count to the lockup failuredecision threshold time T5 at step P13. In this instance, although thethreshold value DS may theoretically assume zero (0), it is establishedto be slightly greater than slippage which possibly occurs due tochanges of the diameters of wheels resulting from wear and other factorswhile the lockup clutch 4 is completely locked. On the other hand, ifthe answers to all of the decisions at steps P9 through P12 are "YES,"the timer TL5 changes its count by a decrement of one at step P14. Afterhaving reset the initial count at step P13 or having changed the timercount at step P14, a decision is made at step P15 as to whether thetimer TL5 has counted down to zero (0). At an early time, since theanswer to the decision is "NO," the lockup failure decision sequenceroutine returns without executing further steps for the lockup failuredecision. When the answer to the decision is "YES," the lockup failureflag LU is set to a state of "1" which actuates the warning device 21 toindicate an occurrence of a lockup failure of the automatictransmission. As described above, when the slippage for the first tofourth gears are estimated to be greater than the threshold value DS forthe lockup failure decision threshold time T5, it is determined thatthere has occurred a lockup failure.

In the case where the answer to any one of the decisions made at stepsP4 through P7 is "NO," the lockup failure decision sequence routineskips steps P8 through P16 and proceeds directly to step P17 in FIG. 7where a decision is made as to whether the lockup failure flag LU hasbeen set to the state of "1". If the answer to the decision at step P17is "NO," this indicates that withdrawal of the result of failuredecision is not necessary, then, the lockup failure decision sequenceroutine returns without executing further steps for the lockup failuredecision. However, if the answer to the decision at step P17 is "YES," adecision is made at step P18 as to whether a timer TL6 has counted downto zero (0). If the answer to the decision is "YES," the timer TL6resets its initial count to the lockup failure decision threshold timeT6 at step P19. On the other hand, if the answers to the decision atstep P18 is "NO," the timer TL6 changes its count by a decrement of one(1) at step P20. After having reset the initial count at step P19 orhaving changed the timer TL6 count at step P20, a decision is made atstep P21 as to whether the timer TL6 has counted down to zero (0). Ifthe answer to the decision is "NO," the lockup failure decision sequenceroutine returns without setting the lockup failure flag LU to the stateof "0". When the answer to the decision is "YES," the lockup failureflag LU is set to the state of "1" at step P22, and the lockup failuredecision sequence routine returns.

FIG. 8 is a flowchart illustrating a sequence subroutine of the decisionas to whether a gear shift is under execution, which is made at step Q10of the failure decision sequence routine shown in FIG. 2. The flowchartlogic commences and control passes directly to a function block at stepQ101 where a decision is made as to whether the current target gearTG.sub.(n) is consistent with the last target gear TG.sub.(n-1). Ifthese target gears are not consistent with, this indicates that a gearshift takes place, in other words, that a shift command signal isoutput, then, a decision is subsequently made at step Q102 as to whethera shift flag SG has been set to a state of "0". In this instance, theshift flag SG is set to a state of "1" while a gear shift is underexecution and set to the state of "0" when there is no occurrence of agear shift. At an early time, the answer to the decision made at stepQ102 is "YES" and the last target gear TG.sub.(n-1) is employed as atarget gear TG at step Q103. Subsequently, after having set the shiftflag SG to the state of "1" at step Q104, a threshold time TS isestablished according to the oil temperature THO, the gear TO and thetarget gear TG.sub.(n) at step Q105. In this instance, the thresholdtime TS is established to be greater as the oil temperature THOincreases and as the difference between the gears TG and TG.sub.(n)becomes larger.

If the answer to the decision made at step Q101 is "YES," a shiftcommand signal is interrupted and a decision is made at step Q106 as towhether the shift flag SG has been set to the state of "1". If theanswer to the decision is "YES," after having changed the count of ashift timer TM by an increment of one (1) at step Q108, a decision ismade at step Q109 as to whether the shift timer TM has counted a timegreater than the threshold time TS. The answer to the decision is "NO,"this indicates that the gear shift has not yet been completed, then, thesequence routine returns. On the other hand, if the answer to thedecision is "YES," this indicates that the gear shift has beencompleted, then, after having reset the shift flag SG to the state of"0" at step Q110, the sequence routine returns. If the answer to thedecision made at step Q106 is "NO," after having cleared or reset theshift timer TM to zero (0) at step Q107, the sequence routine returns.As apparent from the above description, the failure decision sequenceholds the shift flag SG set to the state of "1" for the threshold timeTS even after the interruption of a shift command signal, interruptingthe failure decision until slippage becomes stable.

FIGS. 11 and 12 show another embodiment of the invention in whichvehicle speeds for allowing the failure decision are set in a differentmanner.

FIG. 12 shows characteristic curves of the estimated slippage of thetorque converter in connection with vehicle speeds when the lockupclutch is locked and when the lockup clutch is unlocked.

Considering a case where the third gear is selected as the target gear,the characteristic curve peculiar to the third gear in the case wherethe lockup clutch is locked is intersected by the characteristic curvepeculiar to the fourth gear in the case where the lockup clutch isunlocked at a specific vehicle speed NV2, and the characteristic curvepeculiar to the third gear in the case where the lockup clutch isunlocked is intersected by the characteristic curve peculiar to thesecond gear in the case where the lockup clutch is locked at a specificvehicle speed NV1 and by the characteristic curve peculiar to the firstgear in the case where the lockup clutch is locked at a specific vehiclespeed NV3. In the case where the third gear is selected as the targetgear, since, whenever the vehicle speed is lower than the specific speedNV1, NV2 or NV3 (NV3<NV2<NV1), the estimated slippage is greater for thethird gear than for another specific gear, a wrong failure decisionpossibly occurs. More specifically, the failure decision is correctlyexecuted when the vehicle speed is greater the specific speed NV1 and,however, possibly results in incorrect execution when the vehicle speedis lower than the specific speed NV3. When the vehicle speed is betweenthe specific speeds NV1 and NV3, the failure decision is executedsometimes correctly and sometimes incorrectly. It is of course thatthese specific speeds NV1-NV3 depend upon gears selected as the targetgear.

In this embodiment, although there possibly occurs a wrong failuredecision even when the automatic transmission operates normally, thethreshold vehicle speed for allowing the execution of the failuredecision is established within a range of vehicle speeds greater thanthe smallest speed NV3 in order to give priority to reliable executionof the failure decision. That is, the threshold vehicle speeds NVX1-NVX4(see FIG. 3) for allowing the execution of the failure decision areestablished corresponding to the smallest speed NV3.

In this embodiment, the greatest speed NV1 is employed as a thresholdvehicle speed for allowing the withdrawal of a result of the failuredecision. That is, the withdrawal of a result of the failure decision isallowed only in a range of vehicle speeds greater than the thresholdspeed NV1 where there is no possibility of wrong failure decisions.

FIG. 11 shows part of a flowchart of the failure decision sequenceroutine in which the greatest speed NV1 is employed as a thresholdvehicle speed for allowing the withdrawal of a result of the failuredecision. As shown in FIG. 11, if the answer to the decision concerningslippage made at step Q28 in FIG. 4 is "NO," an additional decisionconcerning the threshold vehicle speed NV1 is made at step Q35B betweensteps Q28 and Q35. Specifically, when the answer to the decision made atstep Q28 as to whether the current estimated slippage SLP_(TG)(n) isgreater than the first threshold value DS1 is "YES," a decision is madeat step Q35B as to whether a vehicle speed NV is greater than thethreshold speed NV1. Only when a vehicle speed is greater than thethreshold speed NV1, the withdrawal of a result of the failure decisionis executed through steps Q35-Q40.

A decision concerning the threshold speed NV1 is also made at step Q49Bbetween steps Q43 and Q49 in FIG. 5. That is, only when a vehicle speedis greater than the threshold speed NV1, the withdrawal of a result ofthe failure decision is executed through steps Q49-Q54.

The threshold slippage may be given not as a difference but as a ratiobetween the engine speed and estimated turbine speed.

The result of a decision that there has occurred a failure in theautomatic transmission may be used to various purposes in place of, orotherwise in addition to, being shown by the warning device. That is,the result may be stored in the ROM of the control unit U in order tomake diagnosis of the automatic transmission at a workshop, or used toexecute a special gear shift control upon a failure or to conduct aspecial control for removing a failure.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

What is claimed is:
 1. A failure decision system for an automatictransmission which is comprised of a torque converter equipped with alockup clutch for locking said torque converter with a given lockupcharacteristic, a multiple speed transmission gear mechanism to whichengine output torque is transmitted through the torque converter andshift means for automatically shifting said multiple speed transmissiongear mechanism to target gears selected according to predetermined shiftpatterns on the basis of driving conditions including at least a vehiclespeed for making a decision of operational failure than a current gearto which the automatic transmission is being shifted by said shift meansis different from a selected target gear, said failure decision systemcomprising:a speed sensor for detecting a vehicle speed; a speed sensorfor detecting an engine speed; failure decision means for calculating aturbine speed of said torque converter for a target gear selectedaccording to said vehicle speed and said selected target gear,calculating an amount of slippage occurring in said torque converter onthe basis of said engine speed and said calculated turbine speed,determining a threshold amount of slippage to which a maximum amount ofslippage possibly occurring in said torque converter is setcorresponding to said vehicle speed and said target gear when saidtarget gear is regarded as having been correctly achieved in saidmultiple speed transmission gear mechanism, comparing said calculatedamount of slippage with said threshold amount of slippage and decidingan operational failure that a current gear to which said automatictransmission is being shifted is different from said selected targetgear based on an occurrence of said calculated amount of slippage inexcess across said threshold amount of slippage.
 2. A failure decisionsystem as defined in claim 1, wherein said failure decision meansfurther interrupts said decision of failure while said torque converterunstably causes slippage.
 3. A failure decision system as defined inclaim 2, wherein said failure decision means further interrupts saiddecision of failure during a gear shift.
 4. A failure decision system asdefined in claim 3, wherein said failure decision means interrupts saiddecision of failure during a presence of a gear shift command.
 5. Afailure decision system as defined in claim 4, wherein said failuredecision means interrupts said decision of failure for a predeterminedperiod of time after a disappearance of said gear shift command.
 6. Afailure decision system as defined in claim 4, wherein saidpredetermined period of time is established to be longer with a declinein temperature of an oil in said multiple speed transmission gearmechanism.
 7. A failure decision system as defined in claim 2, whereinsaid failure decision means further interrupts said decision of failureduring coasting.
 8. A failure decision system as defined in claim 1,wherein said failure decision means withdraws a decision of failure whensaid calculated amount of slippage becomes within said predeterminedthreshold slippage after having decided said failure once.
 9. Thefailure decision system as defined in claim 8, wherein said failuredecision means interrupts said withdrawal of said decision of failurewhen said vehicle speed is lower than a first specified speed.
 10. Thefailure decision system as defined in claim 9, wherein said firstspecified speed is set to a vehicle speed at which a turbine speed for atarget gear while said lockup clutch locks said torque converter isidentical with a turbine speed for a gear lower than said target gearwhile said lockup clutch unlocks said torque converter.
 11. The failuredecision system as defined in claim 8, wherein said failure decisionmeans makes said decision of failure only when said vehicle speed ishigher than a second specified speed.
 12. The failure decision system asdefined in claim 11, wherein said second specified speed is set to avehicle speed at which a turbine speed for a target gear while saidlockup clutch locks said torque converter is identical with a turbinespeed for the lowest gear while said lockup clutch unlocks said torqueconverter.
 13. The failure decision system as defined in claim 1,wherein said failure decision means further decided an occurrence ofsaid operational failure when said calculated amount of slippage becomesmaller than zero.
 14. The failure decision system as defined in claim1, and further comprising lockup clutch control means for controllinglockup and unlock conditions of said lockup clutch according to enginoperating conditions and lockup failure decision means for deciding afailure of said lockup clutch control means based on an occurrence ofsaid calculated amount of slippage greater than a specified amount ofslippage smaller than said threshold amount of slippage while saidlockup clutch control means provides a lockup signal with which saidlock up clutch is locked up.
 15. The failure decision system as definedin claim 1, wherein failure decision means decides a failure that acurrent gear to which said automatic transmission is being shifted isdifferent from said selected target gear based on an occurrence of saidcalculated amount of slippage greater than said threshold amount ofslippage for a predetermined period of time.
 16. The failure decisionsystem as defined in claim 15, wherein said predetermined decision timeis altered according to driving conditions.
 17. A failure decisionsystem as defined in claim 16, wherein said predetermined decision timeis altered according to target gears.
 18. A failure decision system asdefined in claim 17, wherein said predetermined decision time is alteredshorter as said target gear becomes lower.
 19. A failure decision methodof making a decision of operational failure of an automatic transmissionthat a current gear to which the automatic transmission is being shiftedis different from a selected target gear, said automatic transmissioncomprising a torque converter equipped with a lockup clutch for lockingsaid torque converter with a given lockup characteristic, a multiplespeed transmission gear mechanism to which engine output torque istransmitted through the torque converter and shift means forautomatically shifting said multiple speed transmission gear mechanismto target gears selected according to predetermined shift patterns onthe basis of driving conditions including at least a vehicle speed, saidmethod comprising the steps of:detecting a vehicle speed; detecting anengine speed; calculating a turbine speed of said torque converter for atarget gear selected according to said vehicle speed; calculating anamount of slippage occurring in said torque converter on the basis ofsaid engine speed and said calculated turbine speed; determining athreshold amount of slippage to which a maximum amount of slippagepossibly occurring in said torque converter is set corresponding to saidvehicle speed and said target gear when said target gear is regarded ashaving been correctly achieved in said multiple speed transmission gearmechanism; comparing said calculated amount of slippage with saidthreshold amount of slippage; and deciding an operational failure that acurrent gear to which said automatic transmission is being shifted isdifferent from said selected target gear based on an occurrence of saidcalculated amount of slippage in excess across said threshold amount ofslippage.
 20. The failure decision system as defined in claim 19,wherein said failure decision means further decides an occurrence ofsaid operational failure when said calculated amount of slippage becomessmaller than zero.
 21. The failure decision system for an automatictransmission which is comprised of a torque converter equipped with alockup clutch for locking said torque converter with a given lockupcharacteristic, a multiple speed transmission gear mechanism to whichengine output torque is transmitted through the torque converter andshift means for automatically shifting said multiple speed transmissiongear mechanism to target gears selected according to predetermined shiftpatterns on the basis of driving condition including at least a vehiclespeed for making a decision of operational failure that a current gearto which the automatic transmission is being shifted by said shift meansis different from a selected target gear, said failure decision systemcomprising:a speed sensor for detecting a vehicle speed; a speed sensorfor detecting an engine speed; failure decision means for calculating aturbine speed of said torque converter for a target gear selectedaccording to said vehicle speed and said selected target gear,calculating an amount of slippage occurring in said torque converter onthe basis of said engine speed and said calculated turbine speed,determining a threshold amount of slippage to which a maximum amount ofslippage possibly occurring in said torque converter is setcorrespondingly to said vehicle speed and said target gear when saidtarget gear is regarded as having been correctly achieved in saidmultiple speed transmission gear mechanism, comparing said calculatedamount of slippage when said threshold amount of slippage, and makingdecision of operational failure that a current gear to which saidautomatic transmission is being shifted is different from said selectedtarget gear based on an occurrence of said calculated amount of slippagein excess across said threshold amount of slippage only when saidvehicle speed is higher than a second specified speed which is a vehiclespeed at which a turbine speed for a target gear while said lockupclutch locks said torque converter is identical with a turbine speed forthe lowest gear while said lockup clutch unlocks said torque converter.